The advantages of cathodic electrophoretic enameling are described in the technical literature and in patents (Metalloberflache 31 (1977) 10, pages 455-459, EP-A-4 090, AT-C-3 46 989, U.S. Pat. No. 3,922,253, EP-B-66 859).
A disadvantage of the systems described is the use of relative high stoving temperatures of about 170.degree. C., as a result of which the expenditure of much heating energy is required. In the automobile industry, the main user of the cathodic electrophoretic enameling method, these high stoving temperatures prevent the inclusion of plastic parts in car bodies before priming and in some cases reach the melting temperatures of soldered connections.
The suppliers of cathodic electrophoretic enameling systems must therefore be able to produce these materials, with which the previously good properties of the primers are not adversely affected at low stoving temperatures (low bake).
In U.S. Pat. No. 4.443,559, .beta.-hydroxyurethanes are disclosed as cross-linking agents for cathodic electrophoretic enameling systems with a low stoving temperature. At 250.degree. F. (121.degree. C.), curing that is described as "appreciable" is achieved (column 6, lines 12-13). Only at 325.degree. F. (163.degree. C.) is the curing "excellent" and thus usable for car bodies. A further disadvantage is the handling of larger amounts of poisonous isocyanates during the production of this enamel system.
In EP-A-1 25 577, curing at low temperatures is achieved by the combination of onium group-containing polymers with special amino resins, however, at the expense of inferior corrosion protection properties.
The corrosion protection results, achieved at stoving temperatures below 160.degree. C. even with the systems from DE-A-32 46 812 and DE-A-33 28 455, are also inadequate for the demands of the automobile industry.
DE-A-33 46 812 discloses curable, cationic, modified products of epoxide resins, which contain .beta.-hydroxyalkylcarbamide ester groups, while DE-A-33 28 455 starts out from amino acid/salt adducts with urethanized amino groups.
In the DE-A-33 24 211 and DE-A-33 15 469, Michael addition products of monocarboxylate or dicarboxylate esters with compounds, which contain double bonds capable of reacting in a Michael addition reaction, are combined for cross linking with hydroxyl group-containing synthetic resins, for example, with cathodic electrophoretic enameling support resins.
As stoving conditions, a temperature of 130.degree. to 200.degree. C. over a period of 10 to 60 minutes and preferably a temperature of 150.degree. to 180.degree. C. over a period of 15 to 30 minutes are mentioned. In the examples, the stoving temperatures are between 150.degree. and 200.degree. C. Coatings, 16 to 18 .mu.m thick and stoved at temperatures below 150.degree. C., do not fulfill the requirements of the automobile industry, for example, with respect to good mechanical properties, sufficient resistance in the ASTM 117/6 or DIN 50 021 salt spray test or good outdoor exposure resistance.
DE-A-35 30 179 discloses the cross linking of cathodic electrophoretic enameling support resins, which have terminal or lateral double bonds, as well as reactive hydroxyl groups, with Michael addition products of monocarboxylic or dicarboxylic acids, which are capable of participating in a Michael addition reaction and suitable double bond-containing compounds. At stoving conditions of practically 60 minutes at 120.degree. C., corrosion-resistant and robust coatings are obtained.
The EP-A-208 867, in which the cross linking of cathodic electrophoretic enameling support resins, which have hydroxyl groups, with components of the reaction products of diisocyanate with CH-active alkyl esters and polyamines is regarded as a transesterification reaction, also discloses coatings, for which adequate properties can be anticipated only at stoving temperatures above 120.degree. C.